A hybrid vehicle including a motor that operates with electric energy as well as an engine that operates with combustion energy as driving force sources of running of a vehicle has conventionally been known. This hybrid vehicle mainly includes the following types: (1) a series hybrid system in which an engine operates as a power supply source when the engine drives wheels without a motor; and (2) a parallel hybrid system in which wheels are driven by both an engine and a motor. Further, there is another type called a parallel-series hybrid system having both these functions.
Except for the series hybrid system, a motor is used as an auxiliary driving source that assists an engine in outputting. Such a hybrid vehicle performs various kinds of control, such as assistance of a motor to an engine in outputting when the vehicle is accelerating, and charging a battery and the like by deceleration regeneration when the vehicle is decelerating. This can satisfy demands of a driver while securing the residual capacity of a battery. Such a hybrid vehicle includes a power drive unit (referred to also as a “PCU (power control unit)”) for the purpose of drive or regeneration of a motor. The power drive unit includes a plurality of switching elements, and drives a motor or performs regeneration of a motor by current control using the switching elements. The hybrid vehicle further includes a motor control device that outputs a control signal causing the switching elements to perform switching.
A battery that stores electric power supplied to a motor is mounted on the foregoing hybrid vehicle. The motor is connected to an inverter. The inverter is connected to the battery. Provided between the inverter and the battery is an SMR (system main relay) that electrically connects and disconnects the inverter with the battery. The SMR includes a positive SMR provided on the positive electrode of a battery, a negative SMR provided on the negative electrode of a battery, and a precharge SMR that is connected in parallel to the positive SMR and to which a resistor is connected in series. A large-capacity electrolytic capacitor is provided between terminals on an input side of the inverter so as to smooth the fluctuation of voltage to stabilize operation of the inverter. When a hybrid vehicle runs, the capacitor is charged with the main SMRs closed (with the positive SMR and the negative SMR closed) by operating an ignition switch. However, if the capacitor is charged directly by a battery, a large amount of current may flow to cause damage to contacts of SMRs. To address this disadvantage, first, the precharge SMR is closed and the capacitor is precharged until a certain time has elapsed while a current is limited by a limiting resistor or the like. After precharging is finished, the main SMRs are closed. Contacts of SMRs are thus prevented from damage.
Further, electric power may be supplied to an electric load by using a plurality of batteries, and a plurality of batteries may also be charged, which is not limited to the foregoing hybrid vehicle. Even in such cases, the foregoing precharging is needed. Japanese Patent Laying-Open No. 2002-10502 discloses a charge and discharge device for storage batteries that concurrently performs charging and discharging although the device is not limited to one for vehicles. The charge and discharge device for storage batteries includes a switching element to which an ac voltage is inputted and that controls the output, a smoothing capacitor that smoothes the output and applies the smoothed ac voltage to a storage battery, a first voltage detector that detects a voltage across the smoothing capacitor, a second voltage detector that detects a storage battery voltage of the storage battery, and a control device that causes the switching element to control so that a detection signal of the first voltage detector becomes a detection signal of the second voltage detector.
With this charge and discharge device for storage batteries, if there is a difference between a voltage across the smoothing capacitor and a storage battery voltage of the storage battery, the voltage across the capacitor is detected by the first voltage detector and the storage battery voltage of the storage battery is detected by the second voltage detector. Control of the switching element is performed by the control device so that the detection signal of the first voltage detector becomes the detected voltage of the second voltage detector. This eliminates the difference between the voltage across the smoothing capacitor and the storage battery voltage of the storage battery. Charging from the storage battery to the capacitor is thus gradually performed. No excessive current flows.
However, although the foregoing charge and discharge device for storage batteries disclosed in Japanese Patent Laying-Open No. 2002-10502 includes a plurality of storage batteries and includes a plurality of opening/closing means (relay) to suppress occurrence of an excessive current, there is no mention of detecting abnormality (immobility in which an opening/closing operation does not work, fixation (welding) in which a unit remains in the closed state and is not changed to the open state) of the opening/closing means.
In a case of having a plurality of power storage mechanisms and using (charging and discharging) the power storage mechanisms with switchover among them or simultaneously, a plurality of relays are needed. It is very important for hybrid vehicles to detect what relay is abnormal among a plurality of relays and what is the abnormality like.